With the careful use of the polywater episode in the history of chemistry as a case study, I will show that the distinction recently made in the philosophy of science between experimenting on an entity and manipulating that entity is best seen as a distinction between experimenting on, and experimenting with, that entity. The polywater case also reveals that Ian Hacking's 1983 manipulability criterion is not a necessary condition for realism, and that scientists can, and do, justifiably change their minds as to the reality of theoretical entities